While rates of thyroid cancer among men have declined during the 1970s and 1980s in western Washington, rates among women have increased. The increase in incidence is particularly marked for women aged 35-54, in whom rates have nearly doubled from 1974-1987. Exposure to childhood irradiation of the head and neck (the principal established risk factor for thyroid cancer) seems unlikely to fully account for these observations, given the declining prevalence of this exposure among women in this age group. Previous studies suggest that hormonal factors, such as use of exogenous estrogens, parity and hormonal abnormalities associated with miscarriage and infertility may increase risk of thyroid cancer. However, these earlier studies may have considerably underestimated risk associated with oral contraceptive use due to inadequate opportunity for exposure, inadequate years of elapsed latent period, or greater prevalence of radiation-induced cancers. We propose a population-based case-control study of thyroid cancer among women in three counties of western Washington to test the hypothesis that oral contraceptive use, particularly that occurring early in life or for long time periods, may increase risk of thyroid cancer. All white female residents of King, Pierce, and Snohomish Counties aged 18-64 years and diagnosed with incident thyroid cancer of the follicular epithelium from January 1, 1988 through June 30, 1995 will be identified and invited to participate in the study. Cases will be identified through the Cancer Surveillance System (CSS), a population-based tumor registry that has served the area since 1974. Controls will be selected-by random digit dialing and will be frequency matched to cases on age (by five-year intervals) and county of residence. Cases and controls will be interviewed in person regarding their use of oral contraceptives and other exogenous estrogens, as well as reproductive and menstrual history, history of infertility and other known or suspected risk factors for thyroid cancer.